Currently, gas bills, in particular in the domestic and commercial sector, are based exclusively on the gas volume which has been used. Therefore, gas meters which are based directly on measuring the volume of the gas which has flowed through, in some cases compensating for measurement errors which arise through temperature changes, are primarily used.
The gas meter which is most frequently used is the so-called bellows gas meter as described by U. Wernekinck, Gasmessung and Gasabrechnung [Gas measurement and gas billing], Vulkan-Verl., 1996, 20-31. The bellows gas meter has two measuring chambers which are alternately filled and emptied again by the gas flowing through. While one chamber is being filled, it displaces the gas into the other. The filling and emptying steps are counted and, multiplied by the volume of the measuring chamber, result in the overall volume of the gas which has flown through. However, since the volume of the gas varies with changes in the ambient temperature and pressure; these measurements are subject to errors. In summer, when the gas is warm and takes up a larger volume, the consumer would pay more for the same, calorific value of the gas than in winter. For this reason, modern bellows gas meters are provided with simple mechanical or electrical devices for temperature compensation, in practice, however, these are rarely used. However, pressure fluctuations are not taken into account.
WO 99/06800 has disclosed a gas meter which determines a volumetric flow rate. For this purpose, in a gas pipe, a first thermistor detects the cooling behaviour and a second thermistor detects the current temperature of the gas, and a flow rate of the gas molecules is determined from these parameters. A cell in which the cooling behaviour of stationary gas is detected is also arranged in the pipe. Consequently, a calibration value can be obtained at any desired time when the gas pipe is operating. This calibration value can then in turn be used to determine the volumetric flow rate from the cooling behaviour of the first thermistor.
Despite all these compensatory measures, gas meters which are based on volumetric measurements are always prone to errors and lead to an incorrect gas bill. Moreover, a charging principle which is based on volumetric consumption is unfair to the consumer. This is because his gas consumption is determined not by the volume, but rather by the quantity of gas, i.e. the consumed mass of gas, and by the quality of the gas, i.e. its calorific value. The denser and the more high-quality the gas, the less volume is required to achieve the same efficiency, whether in a heating system, a hot-water system or a cooking area.
Therefore, German patent application No. 199 08 664.8, which has not yet been published, describes a gas meter which determines the gas mass flow rate and therefore takes account of the density of the gas. To do this, it is preferable to use an anemometer, as is known from F. Mayer et al., Single-Chip CMOS Anemometer, Proc. IEEE, International Electron Devices Meeting (IEDM, 1997), 895-898. The disclosure of these two documents forms part of the following description.
However, in the gas meters which have been described above, fluctuations in the quality-of the gas are not taken into account. These fluctuations are considerable in particular in the gas of natural gas and arise primarily because the composition of the natural gas differs according to its source. However, in the supply of gas, gases from different sources are supplied in mixed form, and the mixing ratio may vary considerably depending on demand.
It is true that the prior art has disclosed appliances which take account of the calorific value of a gas and determine an energy consumption. For example, WO 00/11465 has disclosed an energy-measuring appliance which on the one hand has a bellows gas meter for measuring the volume and on the other hand has a device for determining the calorific value of a gas, this calorimetric device being based on an acoustic measurement principle. U.S. Pat. No. 6,047,589 has also disclosed an energy-measuring appliance which determines flow volume and calorific value of a gas; in this case, both measurements are based on the acoustic effect. Therefore, both energy-measuring appliances are calibrated for volume measurement, in each case carrying out a calculation using the calorific value which is currently measured on site and the volume measured value to obtain the desired energy value.
These energy-measuring appliances are therefore relatively complicated, having to carry out both a volume measurement and a determination of calorific value and, moreover, having to link the two measured values obtained. Appliances of this type are therefore too expensive for use as standard gas meters in the domestic and commercial sector.